Fibres are often combined with other materials to form composites thereby increasing the mechanical strength of such materials. Fibres may be made of inorganic or organic material or sometimes a combination of the two. Whiskers are sometimes considered to be synonymous with fibres, but it is generally understood that whiskers have smaller diameter and shorter lengths than fibres.
In the following discussion fibres are considered to be elongated substances having chemically substantially homogeneous composition, and having an average diameter and an average length. The ratio of the average diameter of the type of fibre under consideration, to its average length is usually substatially greater than 5. Such ratio is sometimes referred to as aspect ratio. It is considered that a fibre may be a single elongated substance forming a relatively long continuous thread, or it may consist of several shorter fibres spun or stuck together to provide a fibre of more substantial length.
Ceramic materials are characterized by having high melting points, are often refractory and are generally resistant to oxidation and corrosion. Ceramic fibres and whiskers have desirable properties such as high melting point, substantial physical strength in relation to their weight, relatively high modulus, good shape retention, resistance to oxidation, moreover ceramic fibres may often be obtained from relatively inexpensive materials. There are many structural applications where ceramic fibres can be usefully incorporated. For example, ceramic fibres such as alumina fibres, are frequently used for reinforcing materials when properties such as those listed above, are required. The average diameter or core dimension of desirable ceramic fibres range from a few microns, or even a fraction of a micron, to as wide as a millimeter.
In some instances of commercial utilization of fibres high strength combined with low electrical resistivity are required. Some transition metal borides and nitrides such as titanium boride, hafnium boride, zirconium boride, as well as titanium nitride, hafnium nitride and zirconium nitride, are ceramics known to have low electrical resistivity, and hence the above transition metal boride and nitride fibres are suitable and desirable in such commercial applications.
There are conventional methods for obtaining oxide, carbide and nitride fibres, by extruding or spinning ceramic oxide, carbide or nitride based particles carried in a viscous solution or by a low melting point organic substance. The extruded or spun fibres containing ceramic particles are subsequently subjected to heat treatment to evaporate the solvent and/or decompose the organic carrier. The extruded or spun fibres prior to the heat treatment are sometimes referred to as precursor fibres. It is generally observed that the elimination of the carrier substance leaves voids in the ceramic fibres so obtained, thus the coherence of the ceramic fibres produced by conventional methods is usually low, and consequently the mechanical strength and modulus of such fibres are low or only moderate. In order to increase the strength and coherence of ceramic fibres obtained by conventional methods, high temperature sintering, such as in excess of 1700.degree. C., is required. High temperature sintering process steps are likely to increase the cost of production of conventional ceramic fibres substantially.
It is also known to grow ceramic fibres between electrodes in an electrical field, but such methods are unlikely to produce ceramic fibres in lengths and quantities which are required in commercial utilization.
Pyrolysis of organic fibres or similar carbon rich filaments to provide carbon fibres has been practised for several decades. It is known to obtain silicon carbide fibres, for example, by a process in which polycarbosilanes are subjected to pyrolysis.
Yoshiharu Kimura in U.S. Pat. No. 5,061,469, describes a process for producing boron nitride fibres by reacting an amine with a borazine compound. It is also known to obtain a composite fibre by providing a coating of titanium boride on tungsten fibres.
There is a need for a method to produce coherent and substantially pore-free transition metal nitride and boride fibres without the application of expensive high temperature sintering steps, which could be utilized in obtaining the fibres in commercially required lengths and quantities.
By one aspect of the invention described hereinbelow, a method is provided whereby polycrystalline titanium boride, hafnium boride and zirconium boride fibres are obtained by reacting continuous boron trioxide precursor fibres at moderately high temperature, in a non-oxidizing atmosphere, with a transition metal halide and hydrogen, which are optionally carried by an inert gas.
By another aspect of the present invention a method is described whereby polycrystalline titanium nitride, zirconium nitride and/or hafnium nitride fibres are obtained by reacting continuous boron trioxide precursor fibres at moderately high temperature, in a non-oxidizing atmosphere, with a transition metal halide and nitrogen gas, in the presence of hydrogen.